Beyond Intracellular Accumulation of Polyhydroxyalkanoates: Chiral Hydroxyalkanoic Acids and Polymer Secretion

Abstract

Polyhydroxyalkanoates (PHAs) are ubiquitous prokaryotic storage compounds of carbon and energy, acting as sinks for reducing power during periods of surplus of carbon source relative to other nutrients. With close to 150 different hydroxyalkanoate monomers identified, the structure and properties of these polyesters can be adjusted to serve applications ranging from food packaging to biomedical uses. Despite its versatility and the intensive research in the area over the last three decades, the market share of PHAs is still low. While considerable rich literature has accumulated concerning biochemical, physiological, and genetic aspects of PHAs intracellular accumulation, the costs of substrates and processing costs, including the extraction of the polymer accumulated in intracellular granules, still hampers a more widespread use of this family of polymers. This review presents a comprehensive survey and critical analysis of the process engineering and metabolic engineering strategies reported in literature aimed at the production of chiral (R)-hydroxycarboxylic acids (RHAs), either from the accumulated polymer or by bypassing the accumulation of PHAs using metabolically engineered bacteria, and the strategies developed to recover the accumulated polymer without using conventional downstream separations processes. Each of these topics, that have received less attention compared to PHAs accumulation, could potentially improve the economy of PHAs production and use. (R)-hydroxycarboxylic acids can be used as chiral precursors, thanks to its easily modifiable functional groups, and can be either produced de-novo or be obtained from recycled PHA products. On the other hand, efficient mechanisms of PHAs release from bacterial cells, including controlled cell lysis and PHA excretion, could reduce downstream costs and simplify the polymer recovery process.

Synthetic Control of Metabolic States in Pseudomonas putida by Tuning Polyhydroxyalkanoate Cycle

“…Beyond the PHA-driven control over metabolism in Pseudomonas , the availability of P. putida strains displaying high flux through PhaZ provides great potential for biotechnological applications. The ( R )-HA monomers are valuable substrates because they can be used as building blocks for a variety of biotechnological applications, such as synthesis of antibiotics, polyesters, and vitamins ( 40 , 50 , 51 ). Key interrelated aspects to consider for cost-effective, large-scale ( R )-HA production are substrate cost, product purification, and strain fitness ( 50 – 52 ).…”

“…The ( R )-HA monomers are valuable substrates because they can be used as building blocks for a variety of biotechnological applications, such as synthesis of antibiotics, polyesters, and vitamins ( 40 , 50 , 51 ). Key interrelated aspects to consider for cost-effective, large-scale ( R )-HA production are substrate cost, product purification, and strain fitness ( 50 – 52 ). In this work, we provide a set of engineered P. putida strains producing differential amounts of ( R )-HAs that are also higher than ( R )-HAs produced by the wild type.…”

Polyhydroxyalkanoate/Antifungal Polyene Formulations with Monomeric Hydroxyalkanoic Acids for Improved Antifungal Efficiency

“…( R )-3-hydroxyalkanoic acids were produced biotechnologically from the mcl-PHA as a starting material [ 20 , 44 ]. They have been recognized as valuable synthons and molecules with bioactive properties [ 18 , 45 ]. ( R )-3-hydroxyoctanoic acid (C8-OH) was shown to inhibit wide range of bacterial and two fungal strains namely C. albicans and M. gypseum with moderate efficiency and MIC values between 500 and 1000 µg/mL [ 18 ].…”

Using inorganic acids to stop purple phototrophic bacteria metabolism improves PHA recovery at a large scale

“…Another reason can be the disruption of the proton motive force or proton Fig. 4 PHA production based on dry biomass wt.% for each batch of R1 (■) and R2 (•) reactors using formaldehyde as a fixing agent gradient, which may cause disability of the bacteria to produce electron transporters that are necessary to polymerize PHA and depolymerize PHA (phaZ, depolymerase enzyme) [26]. PHA synthesis and degradation can happen simultaneously [26].…”